Composite oxygen ion transport membranes have been proposed for a variety of uses that involve the production of essentially pure oxygen by separation of oxygen from an oxygen containing feed through oxygen ion transport through such membrane. For example, each membrane can be used in combustion devices to support oxy-fuel combustion or for partial oxidation reactions involving the production of a synthesis gases.
In such membranes, the oxygen ion transport principally occurs within a dense layer that allows both oxygen ions and electronic transport at elevated temperatures. The oxygen from an oxygen containing feed ionizes on one surface of the membrane and the resultant oxygen ions are driven through the dense layer and emerge on the opposite side thereof to recombine into elemental oxygen. In the recombination, electrons are liberated and are transported back through the membrane to ionize the oxygen.
Such membranes can employ two phases, an ionic phase to conduct the oxygen ions and an electronic phase to conduct the electrons. In order to minimize the resistance of the membrane to the ionic transport, such membranes are made as thin as practical and are supported on porous support layers. The resulting composite oxygen transport membrane can be fabricated as a planar element or as a tube in which the dense layer is situated either on the inside or the outside of the tube.
An example of a composite oxygen ion transport membrane is disclosed in U.S. Pat. No. 5,240,480 that has a dense layer supported on two porous layers. The dense layer can be formed of an ionic conducting phase that contains yttrium stabilized zirconia and an electronic conducting phase that is formed from platinum or another noble metal. The porous layer adjacent to the dense layer is active and is capable of conducting oxygen ions and electrons. The other porous layer can be yttrium stabilized zirconia or calcium-stabilized zirconia.
U.S. Pat. No. 5,478,444 discloses a two-phase material capable of transporting oxygen ions and electrons. The oxygen ion conducting phase can be a metallic cerium oxide incorporating a yttrium stabilizer and a dopant that can be iron or cobalt. The electronic conducting phase can be a perovskite that contains lanthanum, strontium, magnesium and cobalt or lanthanum, strontium cobalt and iron.
U.S. Pat. No. 5,306,411 discloses a dual-phase membrane having an ionic conducting phase formed from Sc2O3-stabilized zirconia. The electronically conducting phase can be a perovskite material containing, for example lanthanum strontium, iron, chromium and vanadium. The resultant dense layer can be supported on atria-stabilized zirconia.
The problem that exists with all composite oxygen ion transport membranes is one of strength and durability. This problem arises in part due to the high temperatures that occur when such membranes are used in connection with oxygen-fuel combustion and in reactors. Since the dense layer is very thin it must be supported. As a result, there must be a close match between the thermal expansion of the dense layer, its porous support and any intermediate active porous layer. Additionally, a further problem exists when such membranes are subjected to high oxygen partial pressures. High oxygen partial pressures are produced in combustion devices because as soon as the oxygen emerges from the membrane, it is consumed by reaction with the fuel. This results in chemical expansion due to the high reducing environment. Additionally, perovskites, when used as supports, are particularly susceptible to a phenomenon known as “creep” in which the material will fail under prolonged thermal and mechanical stresses.
As will be discussed, the present invention provides a composite oxygen ion transport membrane element that is more robust than the prior art composite membranes discussed above and that is particularly suitable to environments of high temperature and chemical expansion.